The present invention relates to a rotational speed differential type torque transmitting assembly, and also to a differential unit incorporating a rotational speed differential responsive type torque transmitting assembly.
A differential unit with a rotational speed differential responsive type torque transmitting assembly is known from JP No. 61-62642.
This differential unit comprises a rotary casing, and a pinion assembly rotatable with the casing. The pinion assembly includes a pinion carrier and pinions on the pinion carrier. A pair of side gears are in mesh with the pinions. Rotatable with these side gears are a pair of rotary cams, respectively. These rotary cams are arranged in a face-to-face spaced relationship, and have a sleeve disposed therebetween. A partition within the sleeve divides the interior thereof into first and second cylindrical chambers. The cylindrical chambers open toward the cam surfaces of the rotary cams, respectively. A body of silicone oil fills these chamber. A pair of plungers are received in the chambers and slidably engaged with the cam surfaces of the rotary cams, respectively. The partition is formed with an orifice which allows a restricted fluid flow communication between the chambers.
This differential unit has the following problems:
When there is a rotational speed differential between the pair of side gears, the plungers reciprocate to subject the oil to adiabatic compression. Owing to this adiabatic compression, the oil temperature increases to cause an increase in volume of oil. Since the sleeve is arranged radially outwardly with respect to the axis of rotation of the rotary casing, the oil within the sleeve is subject to centrifugal force, resulting in a further increase in volume of oil. As a result, a substantially great internal force builds up, causing deformation of rounded tops of the plungers and damage on the cam surfaces.
This great internal force build-up causes breakage of the seal, resulting in a leakage of oil. This results in a shortage of oil after a long use.
During rotational speed differential, the plungers act on the casing in such a manner as to separate the casing and its cover apart. Since the rotary casing is rotatably supported by side bearings, these bearings are subject to this stress from the casing. As a result, the side bearings become bulky and need increased installation space.
A rotational speed differential type torque transmitting assembly has been proposed by the inventors of the subject matter of the present application and described in U.S. patent application Ser. No. 193,628 (Our Ref.: U019-88) filed on May 18, 1988 or West Gernam Patent Application Ser. No. P 38 632.1 (Our Ref.: G019-88) filed on May 16, 1988. Both of these patent applications disclose the same subject matter. The rotational speed differential type torque transmitting assembly illustrated in FIG. 10 of these applications includes a pressure relief valve using a spring biased ball seated on a valve seat formed with a drain port in order to discharge a portion of hydraulic fluid from an accumulator chamber when a hydraulic pressure in the accumulator chamber increases beyond a predetermined value. More specifically, the spring biased ball is always subjected to the hydraulic pressure within the accumulator chamber. The ball is disengaged from the valve seat to open the drain port to allow discharge of a portion of hydraulic fluid from the accumulator chamber through the drain port when a force acting on the ball and derived in response to hydraulic pressure of the accumulator overcomes the force of the spring. According to this torque transmitting assembly, since the hydraulic pressure responsive type valve is used to close the drain port of the accumulator chamber, a difficulty arises in keeping pressure in the accumulator chamber at an appropriate value high enough to prevent occurrence of cavitation in the body of hydraulic fluid.
When the ball valve is urged against the spring to disengage from the valve seat to allow a discharge of hydraulic fluid past the drain port, an effective pressure acting area which the hydraulic pressure within the accumulator chamber acts through becomes larger than an effective pressure acting area which the hydraulic pressure acts through when the ball is seated on the valve seat. Therefore, a drop in the hydraulic pressure within the accumulator chamber cannot be avoided while the ball disengages from the valve seat. If a stronger spring is used to provide an increased set load so as to keep the hydraulic pressure within the accumulator chamber above the appropriate value even after there occurs such a drop, the hydraulic pressure within the accumulator chamber is increased to a value higher than the appropriate valve when the ball is seated against the valve seat. In this case, the structural strength of a retainer which an accumulator spring bears against needs to be increased. Besides, since the force with which each of the pistons is urged into slidable engagement with the cam surfaces increases, the amount of torque transmitted upon no rotational speed differential increases, tending to induce noises and vibrations. If the spring constant of the spring biasing the ball is increased so as to quickly bring the ball back into engagement with the valve seat, it is very difficult to set a value of the hydraulic pressure within the accumulator chamber when the ball is disengaged from the valve seat to open the drain port since the set load of the spring varies greately in response to a small flex of the spring. Thus, this approach has turned out to be impractical. Another inherent structual problem of this type of valve is that dust entrained in the hydraulic fluid may be interposed between the ball and the valve seat, causing a leak of hydraulic fluid past a clearance formed between the ball and the valve seat.
This clearance may be minimized if the valve seat is made of a material which is more comformable to the ball made of a steel. But this solution is not satisfactory.
An object of the present invention is to provide a rotational speed differential responsive type torque transmitting assembly which is improved such that the above-mentioned problems have been removed or at least alleviated.